1. Field of the Invention
The present invention concerns an optical information recording medium used in optical recording and reproducing apparatus and, more in particular, it relates to an optical disk in which information is written by irradiation of a laser beam and the written information is read also by using the laser beam.
2. Description of the Prior Art
In such an optical information recording medium technique, there is known an optical recording and reproducing system using a method of recording and reproducing information such as video images or voices. Also known is an optical disk of Direct Read After Write (DRAW) type or Write Once type which has a single recording layer, for example, made of a material sensitive to a laser beam and capable of forming pits by sublimation due to absorbing the irradiated laser beam. One type of conventional DRAW optical disks has such a structure that a single dye-recording layer made of photosensitive material such as an organic dye and formed on a transparent substrate.
It has been found difficult to obtain an optical disk comprising a dye-recording layer having satisfactory photosensitive characteristics for a laser beam of a predetermined wavelength only by using a single photosensitive material. It is therefore considered to form a dye-recording layer by mixing two or more photosensitive materials. However, when a dye-recording layer is formed to contain two or more photosensitive materials, it is necessary to prepare a solution including a specific solvent with which two or more kinds of photosensitive materials is compatible, and the solution has to be spin-coated. In addition, it is also necessary to take into consideration such a condition that the solvent does not adversely effect on the substrate, i.e. dissolution, swelling etc. Thus, there is a problem that a severe restriction is present for the combination between the photosensitive material and the solvent, as well as that the degree of freedom for the selection of the photosensitive material is narrowed.
For overcoming such restriction, there has been proposed to laminate on a substrate a plurality of thin dye-recording layers each having a different molecular skeleton (Japanese Patent Application Laid-Open Sho 60-147391; hereinafter referred to as the prior art 1). There has been also proposed to laminate on a substrate a plurality of thin dye layers respectively including the same dyes each having an identical molecular skeleton (Japanese Patent Application Laid-Open Sho 63-252791; hereinafter referred to as the prior art 2).
However, the prior art 1 utilizes that a solubility of dye to a solvent is different from that of another dye. Specifically, a first solution of dye is prepared by dissolving a dye A in a solvent a in which the dye A is soluble to the solvent a but insoluble to another solvent b. This first solution is coated on a transparent substrate and makes a dye layer A by drying. Next a second solution prepared by dissolving a dye B in a solvent b in which the dye B is insoluble to the solvent a but soluble to the solvent b is coated on the dye layer A, thereby forming the laminated thin layers of the dyes A/B on the substrate. Accordingly, this lamination technique involves a problem that when two dyes included in the neighbor laminated layers have a similar solubility to a solvent these dyes can not be laminated.
On the other hand, the prior art 2 discloses an information recording medium in which two information recording medium units of an azuleniun type dye are appended to each other. This technique merely proposes to append two information recording medium units, since there is no effective method for preventing the leaching of dyes from the units upon lamination coating process. It involves a problem that the manufacturing steps are complicated to deteriorate the productivity and increase the manufacturing cost.